Virtual machines (VMs) are software abstractions of physical computer systems. VMs are generally designed to execute a software stack in a manner that is substantially indistinguishable from the manner in which it would execute on physical hardware. The term, “software stack” refers a set of software subsystems or components needed to provide a fully functional system, and commonly includes an operating system (OS) and one or more interacting applications running on the OS. VMs are often used as test beds for software development and in the server arena where a plurality of VMs co-exist on a single physical computer.
Some interface is generally required between guest software within a VM and various hardware components and devices in the underlying physical hardware platform. This interface—which may be referred to generally as “virtualization software”—may include one or more software components and/or layers, possibly including one or more of the software components known in the field of VM technology as “VM monitors” (VMMs), “hypervisors,” or virtualization “kernels.” Because virtualization terminology has evolved over time and has not yet become fully standardized, these terms do not always provide clear distinctions between the software layers and components to which they refer. For example, the term “hypervisor” is often used to describe both a VMM and a kernel together, either as separate but cooperating components or with one or more VMMs incorporated wholly or partially into the kernel itself. The term, “hypervisor” can also refer to all virtualization enabling software, including host executables, device simulators, and drivers. However, the term “hypervisor” is sometimes used instead to mean some variant of a VMM alone, which interfaces with some other software layer(s) or component(s) to support the virtualization. In some systems, some virtualization code is included in at least one “superior” VM to facilitate the operations of other VMs. In other systems, specific software support for VMs may be included in the host OS itself. Finally, functionality that has traditionally been implemented in software could also or alternatively be implemented in hardware or firmware. Thus, the term “virtualization software” may be thought of more broadly as virtualization logic. Although the invention is described below in terms of virtualization software, it may also be implemented partially in hardware or firmware.
When a first user-level software program executing within one physical server communicates with a second user-level software program executing within another physical server, the two software programs typically communicate over a physical or wireless network connection such as a local area network (LAN), a wide area network, (WAN) such as the Internet, or some combination of the two. A network can generally be defined as a technology placing one computing device or system in communication with at least one other computing device or system. A common type of network connection is commonly referred to by the term “TCP/IP” (“transport connect protocol/Internet protocol”) which in turn refers to two software layers of a protocol stack. These layers require data being transmitted be encapsulated into network packets that are typically about 1.5 kilobytes each. Each network packet includes a header containing address and sequence information as well as an indication of the size of the packet. The address defines a destination endpoint, and the sequence information enables a receiver to reassemble the data in proper order, in case the packets are received out of sequence. Once the data is encapsulated into data packets, it is passed to a hardware network layer, such as an Ethernet, which is a physical network adhering to the “IEEE 802.3” standard promulgated by the Institute of Electrical and Electronics Engineers (IEEE). There are also many other network technologies, such as, e.g., token-ring, WiFi, Bluetooth, IEEE 1394 (often referred to by the trade name “Firewire”), USB (Universal Serial Bus), and Fibre Channel, among others, each requiring particular manipulations of the data being transmitted. Because each message must be packaged in one or more packets a significant amount of processing is required for one computer to communicate to another over a network protocol. This additional processing may lead to latencies and reduction in throughput.
Virtual network connections implemented by virtualization software allow applications executing on respective VMs, but the same physical computer, to communicate with one another in the same manner as they would if they were executing natively on separate physical computers. Although no physical network is present, communication over virtual networks still requires breaking up a stream of data into individual network packets. The principal of a virtual network operating within disparate entities within a computer system is well known. An example of this technology is described in U.S. Pat. No. 7,111,086, which was granted to VMware, Inc. on Sep. 19, 2006.
Existing virtualization software is available that permits tools to be installed within a VM that permit communication with some component of the virtualization layer. This mechanism is sometimes referred to as a “backdoor” or “hypercall.” The term “hypercall” is a contraction of the phrase, “hypervisor call.” The hypercall allows communication only between the guest OS and the virtualization layer, and does not provide for communication between applications or components residing on different guest VMs but on the same physical host platform.